The overall goal is to understand the role of limb dynamics in the acquisition of motor skills in infancy. When limbs move in voluntary and involuntary actions, their trajectories are a result of active muscle contractions, the forces of gravity, and the motion-dependent forces that arise from the mechanical interactions of the segments of the limbs, the intersegmental dynamics. Neuroscientists have discovered that these nonmuscle forces can contribute significantly to the motion of limbs and that particular joints appear to be especially responsive to these motiondependent forces. There is strong evidence that harnessing and optimizing dynamics is essential to skill acquisition at all ages. Such biodynamics factors have never been quantified in infants. A systems-functionalist theoretical framework is used to generate questions about the development role of dynamical factors: 1. How are moments of force partitioned among the body segments in newborn infants? 2. What is the effect of posture? 3. How do limb dynamics change with age? 4. How do limb dynamics change with task? 5. What are the underlying patterns of muscle activities in common infant actions? 6. How do perturbations of movements affect limb dynamics? These questions will be answered by collaboration between the motor development laboratory of Thelen and the biomechanical laboratory of Zernicke. Infants in a longitudinal and several cross-sectional studies will have their movements recorded by a computerized motion-analysis system, and kinematic and kinetic measures of limb coordination, control, and distribution of energy will be determined. These studies have important developmental implications: 1. They describe, for the first time, how forces are apportioned among body segments as infants gain skill. 2. They show how dynamical factors interact with other structural and contextual variables to determine motor performance. 3. They provide a principled basis for understanding how information about the individual's body and the external world can be transduced to produce developmental change.